Transimpendance amplifiers (also referred to as current-to-voltage converters) convert and amplify an input current into an output voltage. Best performance and high dynamic range of transimpedance amplifiers is achieved if the input current of the transimpedance amplifier does not comprise a direct current (DC) component. Transimpedance-type amplifiers are frequently used in fiber-optic communication systems to detect and to amplify a signal current from a photodiode which detects an optical data signal. The output signal/output current of the photodiode (which is more or less directly proportional to the illumination light power and therefore more or less directly proportional to the optical data signal) is converted into a voltage signal which is used for subsequent data processing steps. Sensitivity of a receiver in a fiber-optic communication system and the dynamic range thereof is mainly determined by the transimpedance amplifier stage.
The principle of signal conversion in a fiber-optic communication system is illustrated by the simplified circuit diagram of FIG. 1(a). There is a photodiode 2 receiving an optical data signal S having a power P_OPT. The photodiode 2 is coupled to a bias supply voltage BIAS and the photocurrent I_S of the photodiode 2 is coupled to an input of an amplifier 4. A feedback resistor R_F is coupled between the input and an output of the amplifier 4. Accordingly, there is a virtual ground node 6 which is coupled to the input of the amplifier 4. An output voltage U_S is provided at an output node OUT of the transimpedance amplifier. If the gain of the transimpedance amplifier is significantly higher than 1, the transimpedance gain equals the value of R_F and the current in the feedback resistor R_F is equal to the signal current I_S.
However, for optimum performance at highest dynamic range, the input current I_S should be bidirectional. In other words, the input current I_S should flow in and out of the amplifier 4 with preferably the same amplitude. A photodiode 2 however provides a uni-directional current, and accordingly, there is a DC current component in the signal current I_S.
FIG. 1(b) illustrates a time-dependent signal current I_S of a photodiode 2 in a fiber-optic communication system. The signal current I_S alternates between a first current level L0 which is identified with the bit information “0” and a second current level L1 which is identified with the bit information “1”. The current level L0 is greater than zero current, because the light source (typically a laser) which is applied for optical data transmission is not completely switched off, if a “zero” bit is communicated. Both current levels L0, L1 are greater than zero (current), and accordingly, there is an average DC offset current I_AVG in the signal current I_S. Even if the current level L0 for a “zero” bit is set to zero current, there will be an average DC offset current which is equal to half the peak current.
The DC current component in the signal current I_S of the photodiode 2 may be reduced by introduction of an additional current source which is coupled to the input of the transimpedance amplifier and will subtract the average DC current I_AVG from the signal current I_S which is delivered by the photodiode 2. In FIG. 2(a), there is a further simplified circuit diagram of a transimpedance amplifier, wherein an additional current source 8 is coupled to the virtual ground node 6 so as to subtract the average DC current I_AVG from the input signal I_S. The modified signal I_S*, which is equal to the current across the feedback resistor R_F, is shown in the time-dependent simplified diagram of FIG. 2(b). The current level L0, which is identified with bit information “0”, is negative (i.e., below zero current) and the current level L1 (of the modified input signal I_S*) which is identified with bit information “1” is above zero current. Now, the averaged value of the input current I_S* is very close to zero current. However, introduction of the additional current source 8 for subtracting the average current I_AVG from the signal current I_S introduces additional noise. If the transimpedance amplifier is used within a fiber-optic communication system, this may significantly affect the sensitivity of the receiver.
In FIG. 3, there is a more detailed simplified circuit diagram of a preamplifier 10 in a fiber-optic communication system. The pre-amplifier 10 comprises an offset cancellation stage having a dummy transimpedance amplifier 12. The dummy transimpedance amplifier 12 is configured similar to the transimpedance amplifier 14, and it is used to generate a reference voltage which is equal to the output voltage of the transimpedance amplifier 14, if there is zero input current. The dummy transimpedance amplifier 12 and the transimpedance amplifier 14 both comprise an amplifier 4 and a feedback resistor R_F which is coupled between an output of the amplifier 4 and an input thereof. The output of the dummy transimpedance amplifier 12 is coupled to a non-inverting input of a gain stage 16 and the output of the transimpedance amplifier 14 is coupled to an inverting input of the gain stage 16. The gain stage 16 comprises an output OUT+ and an inverting output OUT−. The offset cancellation stage comprises an operational amplifier 18 which is coupled to the output OUT+ and to the inverting output OUT− of the gain stage 16, with a respective one of its inputs. The output of the operational amplifier 18 is coupled to a current source 20 for generation of a DC offset current which is subtracted from the signal current I_S. The signal current I_S of the photodiode 2 is sensed at the input node IN of the pre-amplifier 10. Preferably, the value of the offset current which is generated by the current source 20 is more or less equal to the value of the average DC current I_AVG which is included in the signal current I_S of the photodiode 2.
However, the pre-amplifier 10 in FIG. 3 comprises a dummy amplifier 12 which has a negative impact on the power consumption of the system. Further, the offset cancellation loop not only cancels an offset which is generated by the DC content in the input signal I_S, but also cancels all offsets present in the loop. The DC current which is subtracted by the current source 20 from a signal which is coupled to the input IN may not necessarily be identical to an average DC input current of the photodiode 2. The effect may vary from part to part due to offset caused by mismatch of components. Further, the receiver's sensitivity is influenced by the noise contribution of the current source 20. Consequently, the circuit of FIG. 3 is not a preferred circuit for high sensitivity receivers.
In FIG. 4, there is another pre-amplifier 10 for a fiber-optic communication system. The pre-amplifier 10 comprises a low pass filter comprising a resistor R1 and a capacitor C1 which are coupled in series between a supply voltage line VCC and ground. A node between the resistor R1 and the capacitor C1 provides a supply voltage to the photodiode 2. The pre-amplifier 10 receives an input signal current I_S at the input N. The input signal current I_S is coupled to a transimpedance amplifier 14. The transimpedance amplifier 14 comprises an amplifier 4 and a feedback resistor R_F. Further, there is a unity gain buffer comprising an operational amplifier 5, a resistor R2 and a capacitor C2. The unity gain buffer is coupled between a non-inverting input and an inverting input of a gain stage 16. The output of the transimpedance amplifier 14 is coupled to a non-inverting input of the gain stage 16. The unity gain buffer uses a different amplifier 5 than the transimpedance amplifier 14. The unity gain buffer together with the RC-filter (which comprises the resistor R2 and the capacitor C2) extracts and buffers the common mode voltage at the output 30 of the transimpedance amplifier 14. The unity gain buffer provides a reference for the following gain stage (voltage amplifier) 16.
The pre-amplifier 10, according to FIG. 4, takes advantage of the fact that a bias voltage for the photodiode 2 is provided through an on-chip low pass filter (R1, C1). The average DC current of the photodiode 2 will flow through resistor R1, thus creating a voltage drop across the resistor R1. There is another resistor R3 which is matched to resistor R1. An offset cancellation stage further comprises an operational amplifier 26 which is for controlling a first and a second transistor 22, 24 which act as current sources. The operational amplifier 26 senses a voltage difference between the voltage drop across resistor R1 and the voltage drop across the matched resistor R3. Therefore, it senses the bias current (i.e., the DC current component through the photodiode 2). The output of the operational amplifier 26 is coupled to the gates of the two transistors 22, 24. The current source is controlled, in that the voltage drop across the two resistors R2, R3 is identical. If the two current sources (i.e., the two transistors 22, 24), are matched devices, the transistor 22 will subtract an identical current from the virtual ground node 6 and consequently subtract the DC current component from the input current I_S. In the pre-amplifier stage 10, according to the simplified circuit diagram of FIG. 4, there is no need for a dummy amplifier. However, the photodiode 2 needs to be biased from the on-chip low pass filter (R1, C1), and accordingly, the circuit does not work for externally biased photodiodes.